Thermionic cathode



Oct. 25, 1949. C, N, SMYTH v 2,485,668

THERMIONIC CATHODE Filed D60. 5, 1945 Patented Oct. 25, 1949 THERMIONICCATHODE Charles Norman Smyth, London, England, as-

signor, by mesne assignments, to International Standard ElectricCorporation, .New York, N. Y., a corporation of Delaware ApplicationDecember 5, 1945, Serial No. 632,984 In Great Britain June 23 1944Section 1, Public Law y69D, August 8, v1946 Patent expires June 23, 1964(CL. Z50-12.7 .5)

Claims.

The present invention relates to improved thermionic cathodes forelectron discharge devices.

In thermionic valves, it is sometimes desirable to arrange so thatelectrons are drawn practically only from certain limited areas of thecathode; it may also be necessary to reduce to a minimum the .capacitybetween the cathode and van adjacent `electrode in order to reduce Aasfar as pos- .sible the high frequency losses lcaused by the capacitycurrent.

Thus in .a triode oscillator, the ygrid current should be kept to a lowvalue; and in a moderately high frequency amplier the losses due to thecapacity current may become serious. As the operation frequency israised, additional losses become appreciable due 'to the power factor ofthe dielectric in the cathode coating and to what is called total.emission damping, which is due to the absorption of high frequencyenergy by elec- 'trons which leave the cathode, but which do not travelfurther than the space charge barrier. This latter effect is of secondorder magnitude, but becomes appreciable owing to 'the very large Vtotal.emission obtainable from coated cathodes.

These effects can be reduced by restricting the emitting areas of thesurface of the cathode so 'that the electrons are produced only wherethey can be efciently employed. Attempts have previously been vmade tocarry out this `principle by poisoning the coated surface of the cathodeover the areas where no emission is wanted, or by coa'tingthe cathodeonly over the areas where .emission is required, such as by sprayingthrough a stencil (or even through the adjacent grid electrode'), or bycoating the cathode all over and scraping the coating olf the areaswhere it is not Wanted.

None of these methods have been found to be satisfactory because theyare difficult to apply and are not very effective, and further, in thecase of amplifier tubes, noise is produced owing to the .presence oftemperature limited areas at the fringes of the active parts.

The difficulty has been overcome according to the present Vinvention Vbyproviding an electron discharge device comprising a plurality ofelectrodes including a thermionic cathode having an fe'lectron emissivecoating `covering substantially the whole of its external surface, thesaid surface :being :shaped in .such manner vthat when the normaloperating potentials .are applied to the several electrodes .of thedevice (except that ythe cathode .heating potential is not applied) anelecftric field :is produced in the .device having ra relatively highintensity `immediately adgiacent to specified areas of the cathode fromwhich a strong electron emission is desired when the cathode is 2heated, and a relatively low intensity in the neighbourhood of theremaining areas.

When the device is normally operated, the

cathode is of `course heated, and the electrons emitted therefrom willproduce a space-charge barrier which considerably modifies the strengthof the field at the surface of the cathode, but nevertheless it is.found that the emission and losses are related to the strength of thefield produced when the cathodes are cold.

A thermionic cathode for an .electron discharge device according to theinvention comprises a surface having raised portions separated bydepressions, the whole of the surface being covered over with .anelectron emitting coating, and the raised portions being disposed sothat they correspond with the perforations in an adjoining electrode orwith openings between adjacent yportions of an adjoining electrode., orso that the depressions correspond with the spaces between two or moreadjoining electrodes.

An ,embodiment of the invention is illustrated in the accompanyingdrawing, in which Fig. 1 shows a .perspective view of the electrodes ofa triode valve having an indirectly heated cathode .according to theinvention, the electrodes being partly 'broken away so that theconstruction can be seen. Fig. 2 shows part of a longitudinal sectionthrough the cathode and control grid of the valve shown in Fig. l.

The electrodes of the valve comprise a corrugated indirectly heatedcathode l, according to the invention, enclosing a heating filament 2, ahelical control grid comprising wires 3 wound on longitudinal supportingwires 4 in a conventional manner, and a cylindrical anode '5. Thecathode comprises a metal tube or vcylinder 1i on which is 'helicallywound a wire '1 having the same 4pitch as the grid. 'The wire l isrigidly attached to the cylinder .6, such as by welding, or by brazingwith a metal of high melting `point such as copper.

The corrugated cathode so formed is then sprayed or otherwise coveredall over with an electron emissive coating indicated by the dottedoutline .8 in Fig. 2.

'The cylinder '6 'and Ywires 'l are preferably of nickel and may bebrazed together with copper at the points 9 in Fig. 2. This may be ydoneby plating the cylinder and/or the wire with copper .before winding onthe wire, and then heating them in an atmosphere free from oxygen untilthe .copper melts, 'thu-s securing them firmly together. The emissivecoating can then be sprayed all over the surface filling up the cornersbetween the wires and the cylinder so as to have a profile somethinglike the dotted line 8.

'The cathode so formed is adjusted in the valve tallic portions of theelectrode.

so that the wires 'l come opposite the spaces between the wires 3 asindicated in Fig. 2, when the cathode is heated to the operatingtemperature. The pitch of winding of the helix of the wire l should beadjusted so that when the cathode is hot, the pitch (which may change asa result of expansion on heating) is the same as that of the grid wires3.

The corrugated surface of the cathode pro- 'duced in this way has theadvantage that the crests of the surface are in a relatively strongelectric field, and the troughs, being further from the grid wires, arein a weaker eld. The electrons are accordingly emitted mainly from thecrests, and the greater accumulation of the active material in thetroughs forms a reservoir of metallic ions which can feed the crests bydiffusing along the surface of the cathode and thus prolong the life ofthe cathode. Another advantage of this form of cathode is that the eX-istence of the troughs opposite the grid Wires means that the capacitybetween the cathode and grid is reduced. The emissive surface of anordinary equivalent plain cathode would need to be about level with thecrests, so that it would effectively be much closer to the grid, andwould have a higher capacity thereto.

Although the valve described for illustration has a helical grid, thecathode design can be modified to suit any type of grid, such as a meshgrid, or one of squirrel cage form, or a flat grid formed of parallelwires or a mesh. Whatever the form of the adjoining electrode, thecathode surface should have a number of raised portions corresponding toand arranged opposite to the spaces or perforations in the electrodethrough which the electrons are to pass, the raised portions beingseparated by depressions which are placed opposite the correspondingwires or Ine- The raised portions may be provided by attaching a wire orwires suitably to the surface of the cathode in the manner exemplifiedin Fig. 1, but there are other ways in which the cathode could beconstructed. For example, it could be formed by just corrugating a thinplain tube. Alternatively, the base of the cathode could be mouldedceramic material having the desired external form, with a metallic filmdeposited on the surface. This form is particularly suitable when theadjacent electrode is a mesh grid. The base of the cathode might be ametal tube on which a suitable groove or grooves are cut or rolled. Thecathode base formed in any of these ways is nally covered with electronemissive material.

A cathode of the kind described may be provided in any type ofmulti-electrode device having two or more grid electrodes. It may beprovided with suitable means whereby its position -with respect to theneighbouring electrode may .be adjusted after assembly, in order toensure accurate registration of the corrugations with the spaces of theelectrode,

A cathode of this kind is also applicable to multiple diodes notcontaining any grid electrodes. In this case there will be two or mo-replate electrodes arranged opposite the cathode, and the requirement isthat the electrons shall be emitted in large quantities opposite eachplate but in small quantities opposite the spaces between the plates.lThe cathode according to the invention will accordingly have raisedportions facing the plates separated by depression registering with thespaces between the plates.

In the following claims, the term perforated electrode means anyelectrode having solid portions separated by spaces through whichelectrons can be passed, and includes a wire wound helical electrode.

What is claimed is:

1. An electron discharge device comprising an indirectly heatedthermionic cathode of ribbed tubular form, a grid electrode mountedadjacent said cathode, and an electron emissive coating coveringsubstantially fthe whole of the surface of said cathode adjacent saidgrid electrode, said ribbed portions of said coated cathode surfacebeing spaced close to said grid, the remaining portions of said coatedcathode surface being spaced at a relatively greater distance from saidgrid than the spacing of said ribbed portions whereby when normaloperating potentials are applied to the device an electric field ofrelatively high intensity is produced immediately adjacent saidspeciiled portions from which a strong electron emission is desired andan electric eld of relatively low intensity is produced immediatelyadjacent said remaining portions from which a weak electron emission isdesired.

2. An electron discharge device comprising a grid electrode consistingof turns of wire and an indirectly heated cathode mounted inside saidgrid electrode, said indirectly heated cathode including a tube of givendiameter having turns of wire secured to its external surface and acoating of electron emissive material covering the said tube and turnsof wire, the said turns of Wire being of small diameter compared withsaid given diameter and being positioned opposite the spaces betweencorresponding wire turns of the grid electrode.

3. An electron discharge device comprising a wire wound grid electrodesurrounding an indirectly heated thermionic cathode consisting of ametal tube of given diameter having separated turns of wire secured toits external surface and a coating of electron emissive materialcovering the said tube and turns of wire, the said turns i of wire beingof small diameter compared with said given diameter and being positionedopposite the spaces between corresponding wire turns of the gridelectrode.

4. A device according to claim 2 in which the said cathode comprising abase of moulded ceramic material having a metallic lm deposited on thesurface thereof.

5. An electron discharge device according to claim 2, in which the saidtube and the said turns of Wire secured thereto are of nickel platedwith copper whereby the said wire turns may be secured to the tube bybrazing.

CHARLES NORMAN SMYTI-I.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,892,819 Van Gessel Jan. 3, 19331,914,883 Cottrell June 20, 1933 1,946,603 Von Wedel Feb. 13, 19342,015,417 Weiller Sept. 24, 1935 2,130,280 Knoll Sept. 13, 19382,130,281 Knoll Sept. 13, 1938 2,210,761 Hennelly Aug. 6, 1940 2,358,542Thompson Sept. 19, 1944 2,380,525 I-Iirman July 31, 1945

